FLUID FLOW - PUMPS
4
1
3
2
Control valve
Suction
Section
Discharge Section
Two main types of pumps:
• Positive Displacement pumps
• Centrifugal pumps
ChE 4253 - Design I
FLUID FLOW - PUMP PERFORMANCE
Mechanical Energy Balance:
4
1
gΔZ + ∫
⎛V 2 ⎞
+Δ⎜⎜ ⎟⎟ = Wo − ∑ F
ρ
⎝ 2 ⎠
dp
3
2
Control valve
Between points 1 and 2 (note that these are 1 and 4 in
previous slide):
⎛ p2 p1 ⎞ ⎛ V2 2 − V12 ⎞
⎟ = ΔWo − ∑ F
g (Z 2 − Z1 ) + ⎜⎜
− ⎟⎟ + ⎜⎜
⎟
ρ
ρ
2
1⎠ ⎝
⎝ 2
⎠
Divide by g to get units of length.
2
2
⎛
p1 V1 ⎞
p2 V2 ⎞ ⎛
⎜ Z2 +
⎟ − ⎜ Z1 +
⎟ = Δh − h f
+
+
⎜
⎟ ⎜
⎟
2
2
ρ
ρ
g
g
g
g
2
1
⎝
⎠ ⎝
⎠
ChE 4253 - Design I
FLUID FLOW - PUMP PERFORMANCE
2
2
⎛
⎞
⎛
p1 V1 ⎞
p2 V2
⎜ Z2 +
⎟ − ⎜ Z1 +
⎟ = Δh − h f
+
+
⎜
⎟
⎜
⎟
2
2
ρ
ρ
g
g
g
g
2
1
⎝
⎠ ⎝
⎠
Z: Potential or static Head
P/ρg: Pressure head
2
V1
: Velocity head
2g
Δh: Total head of the pump
hf: Head loss due to friction
Note: Total head of the system: Solution of the MEB eqn.
Total head of the pump: The manufacturer provides it.
ChE 4253 - Design I
FLUID FLOW - PUMP PERFORMANCE
Total Dynamic Head, TDH
Pd = discharge pressure
Ps= suction pressure
TDH = Pd − PS
Hydraulic Horsepower
Q ( gpm)TDH ( psi )
Wh (hp ) =
1714.3
Shaft Efficiency
η=
Wh
Wb
Wh = theoretical required power (hp)
Wb= actual shaft work or brake-horsepower
Note: η < 1 because there are friction losses inside the pump.
ChE 4253 - Design I
FLUID FLOW - PUMPS
Positive Displacement pumps
Used in cases when large pressure heads are needed.
1. Rotary pumps (rotating gears, lobes or screws).
2. Reciprocating pumps (pistons).
3. Miscellaneous (e.g. peristaltic pumps).
ADVANTAGES:
• Self-priming.
• Can work in two directions.
• Can pump liquids with gases for a small amount of time.
DISADVANTAGES:
• Most of them cannot operate with closed discharge.
• Might produce oscillations in discharge.
ChE 4253 - Design I
ChE 4253 - Design I
ChE 4253 - Design I
ChE 4253 - Design I
ChE 4253 - Design I
FLUID FLOW - PUMPS
Centrifugal pumps
1. Volute pumps (shell and simple impeller).
2. Diffuser pumps (diffuser vanes around the impeller).
3. Turbine pumps.
4. Propeller pumps.
ADVANTAGES:
• Low cost - easy maintenance.
• Do not produce a lot of noise.
• Uniform discharge (no oscillations).
DISADVANTAGES:
• Do not produce large heads.
• Do not work well with high viscosity fluids.
ChE 4253 - Design I
ChE 4253 - Design I
ChE 4253 - Design I
Centrifugal pumps- Advantages and dissadvantages
Advantages
Simple construction
Quiet operation
Long life-time
Capable of pumping
fluids with particles
Capable of pumping
fluids with gases
or vapors
Self-priming
No need for
variable rpm motor
Volute
pumps
X
X
X
X
Diffuser
pumps
X
X
X
Turbines
X
Propeller
pumps
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Disadvantages
Not good for high
viscosity fluids
Low discharge head
Accurate machining
for rotor-shell required
ChE 4253 - Design I
Fluid Flow - Pumps
Available Net Positive Suction Head
PS PV
NPSHA =
−
ρg ρg
Ps= suction pressure
Pv= vapor pressure of fluid
NPSHA has to be positive. Otherwise, the fluid enters
the pump with bubbles.
ChE 4253 - Design I
Fluid Flow - Pumps
NPSHA:
As pressure increases inside the pump the bubbles
collapse.
This phenomenon is called CAVITATION and it
-Reduces capacity
-Damages the pump
ChE 4253 - Design I
Fluid Flow - Pumps
Net Required Positive Suction Head (NPSHR)
Ideal pumps will not cavitate if NPSHA is positive.
However a small pressure decrease can take place in a
pump due to internal losses close to the suction.
====> if NPSHA = 0 bubbles can form and cavitation
takes place.
====> NPSHR is a required value suggested by the
manufacturer
Specification Criteria
NPSHA > NPSHR
ChE 4253 - Design I
Fluid Flow - Pumps
Head Capacity Curves
head
(Pd-PS)
Rotary
Reciprocating
Centrifugal
Positive displacement
pumps give an approximate
constant flow
Flowrate
(gpm)
ChE 4253 - Design I
Fluid Flow - Pumps
Thus, centrifugal pumps are chosen because they can
operate in a wider range of flowrates (better control
and process flexibility).
-------------------------------------------------------------If you are stuck with a positive displacement pump, the
following diagram shows how you can regulate flow.
However, this arrangement will:
- use more energy
- heat up the fluid
ChE 4253 - Design I
Fluid Flow - Pumps
Centrifugal Pump Performance Curves
NPHSR
Δh
Want to operate
somewhere here
6”
Efficiency
70%
60%
8”
Characteristic
Curve
Impeller size
6”
ChE 4253 - Design I
8”
Q (flow rate)
Specifying a Pump
Parameters you can control when selecting the pump:
impeller diameter, speed (not very common), the model
Things to look for: Maximum efficiency, NPSHA > NPSHR
Specifying a Pump
4
1
3
2
Control valve
Need to calculate:
System head
HS = -[(P4-P3)+(P2-P1)]
Pump head:
Hp
Hs
friction
losses
But: Hp = Hs
Static
head
ChE 4253 - Design I
As flow increases
friction losses
increase
Flow rate
Specifying a Pump
Hs
Valve closes
Flowrate
Effect of throttling a valve.
ChE 4253 - Design I
Specifying a Pump
H
Pump curve
System curve
Max. flow in
this interval
Flowrate
Since Hs = Hp , pick the Hs curve close to 80% open,
at maximum flow
What to do if NPSHA is too low
NPSHA = (P2-PV)/rg
1
2
Increase P2 !!!
Increase Z1
(How?)
This is the reason why pumping fluids that are close to
saturated conditions require that the vessel upstream
be elevated. Flash tanks are typical examples of this.
Pumps that return flow from the reboiler to distillation
columns need to be bellow the column level.
ChE 4253 - Design I
Fluid Flow - Pumps
Pumps in parallel:
Total head:
Total flow rate:
Δhtot=Δh1=Δh2
Qtot=Q1+Q2
Pumps in series:
Total head:
Total flow rate:
ChE 4253 - Design I
Δhtot=Δh1+Δh2
Qtot=Q1 = Q2
REFERENCES
Pumps
•McCabe, W.L., Smith, J.C., and P. Harriott, “Unit Operations
of Chemical Engineering,” 5th edition, McGraw-Hill, New
York, 1993. (good for pumps and other unit ops equipment)
ChE 4253 - Design I